# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: Hello (+ a few questions)
From: Thomas Schmidt
Date: 1999 Feb 08, 09:08 EST

R.H. van Gent wrote:
...
> 1) I am trying to locate an obscure reference cited in Bowditch?s
> _American Practical Navigator: An Epitome of Navigation_ (p. 423, 1958
> edition; vol. II, p. 546, 1977/81 edition) to a study performed by the
> Carnegie Institute of Washington on the application of the standard
> formula for the correction for dip:
...
Dear Mr. van Gent,
I have taken the liberty of forwarding your question on HASTRO-L to this
list last Friday, and I received this reply which I'll simply repeat
here
since you've joined us now:
Richard Langley wrote:
> For a good discussion of astronomical refraction, including horizontal
> refraction, see Chapter 15 of Jean Meeus's Astronomical Algorithms.  He gives
> as one reference a paper by Bennett: "The Calculation of Astronomical
> Refraction in Marine Navigation" in the Journal of the (U.K.) Institute of
> Navigation, Vol. 35, 1982, pp. 255-259.
> -- Richard Langley
>    Professor of Geodesy and Precision Navigation
>
> On Fri, 5 Feb 1999, Tom McHugh wrote:
>
> >R.H. van Gent wrote: (and others as well have stated substantially the
> >same thing regarding dip)
> >>
> >> The small angle between both horizons is known as the 'dip', and can be
> >> approximated by the following relation found in almost any astronomical
> >>
> >>   dip (minutes of arc) = 0.97 sqrt(h[ft])
> >>
> >> with 'h' denoting the height of the observer?s eye above sea level in
> >> feet.
> >
> >I think, that for beginners on the list, to avoid confusion, it
> >would be well to state clearly that the above dip formula refers to
> >the sea level as being one's local horizon of reference. and relates to
> >one's
> >vertical elevation of eyepoint above sea level. It must be pointed out
> >that this formula will not be correct if one is on a horizontal plane
> >at some considerable distance above sea level where the local horizon is
> >also
> >well above sea level. Put another way, someone living in Denver or
> >other high plateau regions would have to calculate dip based upon
> >height above the local horizontal plane, not referred to sea level.
> >I am of course, referring to that portion of dip which is related to
> >atmospheric refraction. Naturally, the eye level position above the
> >horizontal
> >will be the same.
> >
> >Even at "sea level" there would be differences in dip, as it has been
> >determined that there are areas of the ocean's surface which are below
> >mean sea level because of mass concentrations within the earth's crust
> >or mantle.
> >
> >Tom McHugh
> >
> >tbmchugh@XXX.XXX
> >
>
>
> ===============================================================================
>  Richard B. Langley                            E-mail: lang@XXX.XXX
>  Geodetic Research Laboratory                  Web: http://www.unb.ca/GGE/
>  Dept. of Geodesy and Geomatics Engineering    Phone:    +1 506 453-5142
>  University of New Brunswick                   Fax:      +1 506 453-4943
>  Fredericton, N.B., Canada  E3B 5A3
>      Fredericton?  Where's that?  See: http://www.city.fredericton.nb.ca/
> ===============================================================================
>
<PRE>
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Thomas Schmidt                    e-mail  : schmidt@XXX.XXX
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